The present invention relates to a powder apparatus for a sheet-fed rotary printing press which sprays powder onto the printed surfaces of sheets, at a delivery unit in the sheet-fed rotary printing press, before the sheets are sequentially stacked on a pile plate.
At the delivery unit in the sheet-fed rotary printing press, sheets that have just been printed are stacked one by one, so the printed surface of a previously piled sheet and the lower surface of a sheet to be piled next come into contact with each other. The sheets to be piled have just been printed and their ink is not sufficiently dried yet. Thus, ink on the printed surface of the previously piled sheet may be undesirably transferred to the lower surface of the sheet to be piled next, that is, a printing trouble so-called blocking (offset) may occur. In order to prevent this, at the delivery unit of the sheet-fed rotary printing press, powder is sprayed onto the printed surface of the sheet, that has just been printed, from the nozzle of a sprayer. The sprayed powder forms a gap between the sheets, thereby preventing blocking.
In this case, if the powder in a quantity more than necessary is sprayed toward the sheet, the excessive powder may be scattered around to attach to the machine, or even worse cause malfunction of the machine. As excessive powder spraying decreases the value itself of the printed product, the quantity of powder to be sprayed toward the sheet is desirably controlled to the necessary minimum. To set the optimal quantity of powder by reducing the spray quantity, the powder must be sprayed uniformly over the entire sheet, and while considering conditions such as the printing speed, printing density, image area ratio, paper quality, sheet stacking count, and the like.
Japanese Patent Laid-Open No. 4-16351 (reference 1) discloses a powder sprayer which sprays powder only in an optimal quantity. The powder sprayer disclosed in this reference has a powder spray quantity initial setting means for setting an initial powder spray quantity, a powder spray quantity detection means for detecting the powder spray quantity, a powder spray quantity adjusting means for adjusting the powder spray quantity of a powder spray means, and a control means. The control means has an initial preset value converting storage which stores in advance a storage table used for converting printing specification data, input and set by the powder spray quantity initial setting means, into the initial preset value of the powder spray quantity, a comparative determination unit for comparing the obtained initial preset value with the actual measurement value of the powder spray quantity detected by the powder spray quantity detection means, and a powder spray quantity controller for controlling the powder spray quantity adjusting means on the basis of a comparative determination signal sent from the comparative determination unit.
In this arrangement, the actual measurement value of the powder spray quantity detected by the powder spray quantity detection means and the initial preset value are compared. The powder spray quantity is adjusted on the basis of the comparison result.
The powder sprayed toward the sheet surface does not entirely attach to the sheet. More specifically, about ⅓ of the powder spray quantity attaches to the sheet, while the remaining ⅔ is scattered around. The ratio of powder attaching to the sheet changes depending on the temperature and humidity and is not always constant. In other words, the quantity powder sprayed from the powder spray means and the quantity of powder actually attaching to the sheet differ. This difference is not constant but changes from time to time. Therefore, with the conventional powder sprayer for controlling the powder spray quantity on the basis of the quantity of powder actually sprayed from the powder spray means, an optimum quantity of powder cannot be attached to the sheet.
Regarding the powder attaching state, the powder attaches to the entire sheet surface not always uniformly but with a certain degree of nonuniformity. When the powder attaching state varies in this manner, in order to prevent blocking, control operation must be performed with reference to a portion with a small attaching quantity as the criterion. Then, the total powder spray quantity increases more than necessary. In particular, when a plurality of nozzles constituting the powder spray means are provided in the widthwise direction of the sheet, the spray quantities of the plurality of nozzles also vary, further increasing the powder spray quantity.
It is an object of the present invention to provide a powder apparatus for a sheet-fed rotary printing press, which sets the spray quantity of powder for blocking prevention to an optimal value.
It is another object of the present invention to provide a powder apparatus for a sheet-fed rotary printing press, which eliminates variation in the spray quantity of powder for blocking prevention and can spray the powder uniformly over an entire sheet.
In order to achieve the above objects, according to the present invention, there is provided a powder apparatus for a sheet-fed rotary printing press, comprising spraying means for spraying powder onto a printed surface of a sheet-like object delivered after printing, adjusting means for adjusting a quantity of powder sprayed from the spraying means, detection means for detecting a quantity of powder attaching to the sheet-like object, and control means for controlling the adjusting means on the basis of data output from the detection means and indicating a powder quantity.